I mean typically either technical or associated with geek culture.
The Big Bang Theory tends to use physics, maths and to a lesser extent computers for technical material, but there are plenty of other areas from U.S. politics (which I've never understood the fascination of to non-Americans - sure, a superficial awareness of it is relevant to anyone, but I know people who can name the order in which the Presidential primaries take place, voting procedures in the Senate, etc.) to evolution to philosophy. Often we're talking about very narrow subsets of these topics.
From geek culture, The Big Bang Theory tends to use science fiction and fantasy, particularly comic book superheroes, but again there's a whole spectrum from chess to art-house cinema.
The thing is, introverts aren't exactly all nerdy. How many of you know someone who is pretty quiet, but can talk for hours about football? How many of you are that someone?